Header alt tags (#1138)

* Added support for an alt tag for the header image

This allows using page.header.image_description as the alt tag. It
will still default to site.title if unset.

* Added documentation

docs/_docs/10-layouts.md

@@ -273,6 +273,14 @@ header:
   image: http://some-site.com/assets/images/image.jpg
 ```
 
+To provide a custom alt tag for screen readers:
+
+```yaml
+header:
+  image: /assets/images/unsplash-image-1.jpg
+  image_description: "A description of the image"
+```
+
 To include a caption or attribution for the image:
 
 ```yaml